Deutsches Zentrum Microwave and Radar Institute für Luft und Raumfahrt e.V. Department Reconnaissance and Security

Operational Aspects of Orbit Determination with GPS for Small Satellites with SAR Payloads Sergio De Florio, Tino Zehetbauer, Dr. Thomas Neff Phone: +498153282357, [email protected]

Abstract Requirements Scientific small satellite missions for remote sensing with Synthetic Taylor expansion of the phase Φ of the radar signal as a Aperture Radar (SAR) payloads or high accuracy optical sensors, pose very function of time varying position, velocity and acceleration: strict requirements on the accuracy of the reconstructed satellite positions, velocities and accelerations. Today usual GPS receivers can fulfill the 4π 233 Φ==++++()t Rtap ()()01kk apttaptt ()(-) 02030 ()(-) k aptt ()(-) k ο () t accuracy requirements of this missions in most cases, but for low-cost- λ missions the decision for a appropriate satellite hardware has to take into Typical requirements, for 0.5 to 1.0 m image resolution, on account not only the reachable quality of data but also the costs. An spacecraft position vector x: analysis is carried out in order to assess which on board and ground equipment, which type of GPS data and processing methods are most −−242 appropriate to minimize mission costs and full satisfying mission payload x≤≤⋅≤⋅ 15 mmsms x 1.5 10 / x 6.0 10 / (3σ ) requirements focusing the attention on a SAR payload. These are requirements on the measurements, not on the real motion of the satellite

Required Hardware Typical Position Accuracies GPS hardware Raw data based POD strategies Manufacturer Receiver Channels / Frequencies / Codes Mission Magnavox GPSPAC 2 / L1, L2 / C/A, P Landsat4, Landsat5, Motorola GPSDR 6 / L1, L2 / C/A, P EUVE, TOPEX/Poseidon, OREX Data Type Processing Scheme Accuracy (m) TANS 6 / L1 / C/A Space Shuttle, PoSat-1, FASat-Alfa, GANE / STS-77, ORSTED TANS Quadrex 6 / L1 / C/A RADCAL ORBCOMM-FM1, ORBCOMM-FM2, Skipper, YES (sub-satellite of Trimble Navigation TANS II 6 / L1 / C/A TEAMsat satellite), FASat-Bravo, ABRIXAS Single frequency PR Kinematic 9.1 APEX, CRISTA-SPAS, GADACS / SPARTAN OAST Flyer, JAWSAT, TANS Vector 6 / L1 / C/A AMSAT Phase 3D, TSX-5, OSEM, EarlyBird, Gravity Probe B Alcatel Alcatel/SEL 6 / L1 / C/A ORFEUS-SPAS-1 Single frequency SPP Reduced-dynamic 0.8 Ashtech SB24 24 / L1 / C/A COMET Ashtech G12 12 / L1 / C/A SEDSat-1 Single frequency PR Reduced-dynamic 0.8 Hitachi GPSR 5 / L1 / C/A SFU OrbView-1 (formerly MicroLab-1), Wake Shield Facility-02, Wake Shield Allen Osborne Associates, Inc. TurboStar 8 / L1, L2 / codeless Facility-03, GFO, ORSTED, SUNSAT MSTI-3, MOMS-2P, OrbView-2 (formerly SeaStar), Equator-S, QuickBird, Single frequency PR & CP Reduced-dynamic 0.3 General Dynamics Viceroy 12 / L1 / C/A EarlyBird, QuikSCAT Space Systems/Loral Tensor 9 / L1 / C/A SSTI Lewis, Globalstar, OSEM, SAC-C, ESA/ATV Navsys TIDGET 8 / L1 / C/A Falcon Gold Dual frequency PR Kinematic 2.9 JPL MicroGPS 12 / L1 / C/A SNOE SGR-10 24 L1 C/A TMSat-1 Surrey Satellite Technology Dual frequency SPP Reduced-dynamic 0.3 SGR-20 24 L1 C/A UoSAT-12 Rockwell - Collins Avionics and AST-V 6 (4 cont., 2 seq.) / L1 or L2 / C/A or P DARPASAT, TAOS/STEP-0, STEP-2 Communications Division Dual frequency PR Reduced-dynamic 0.2 Rockwell Collins GEM-S 5 / L1 / C/A or P BIRD AstroNav JPL/Spectrum Astro 48 / L1, L2 / C/A, P-codeless SRTM, STRV-C, SAC-C, CHAMP, Jason-1, VCL, GRACE, FedSat-1, ICESat (BlackJack) LABEN Co (Italy) Lagrange 16 / L1, L2 / C/A SAC-C Dual frequency PR & CP Reduced-dynamic 0.1 ESA GNSS 12 / L1, L2 / C/A, P MetOp-1 Mayflower Communications Mayflower receiver 12 / L1 / C/A NASA/STV Company, Inc PR: pseudorange, CP: carrier phase, SPP: single point positions

EADS Astrium Mosaic GPS/GNSS 8 / L1 / C/A On-board data sampling and storage Navigation solutions based POD strategies Data Type Processing Scheme Accuracy (m) MB/hour to be stored Data Type Sampling Rate on-board Navigation solutions Kinematic 16.5 Navigation Navigation solutions Reduced-dynamic 1.6 solutions and 1.0 Hz 1.0 Achievable position accuracies for the CHAMP satellite ancillary data Raw data and 1.0 Hz 5.0 ancillary data

Orbit Determination Software Commercial: FreeFlyer by a.i. Solutions (NS), OD Tools Kit by Freeware: GAMIT/GLOBK, GIPSY-OASIS II, AGI (NS &. PR), Bernese Software (NS Raw Data) Trimble Geomatics (TGO)

GPS navigation-solution based orbit determination with FreeFlyer COTS product Accuracy Position 1.0 m Velocity 10-3 m/s Acceleration 10-6 m/s2

The requirement on the acceleration can be fulfilled with the only use of the navigation solutions without any margin and without a 3σ standard deviation.